In recent years, organic EL lighting has been drawing attention as an eco-friendly green device. Features of the organic EL lighting include: 1) a low power consumption as compared with an incandescent lamp; 2) a thin profile and light weight; and 3) a flexibility. At the moment, the organic EL lightings are being developed so as to attain the above features of 2) and 3). In this respect, glass substrates that have been conventionally used in flat panel displays (FPD) are unable to attain the above features of 2) and 3).
In view of this, there are researches being conducted on a substrate as a support (hereinafter, “supporting base material”) for organic EL lighting, proposing an ultra-thin glass, a resin film, a metal foil or the like as a possible supporting base material. The ultra-thin glass is superior in heat resistance, barrier performance and optical transparency and has good flexibility, but is somewhat inferior in handling and has low thermal conductivity and high material cost. Further, the resin film is superior in handling and flexibility and has low material cost and good optical transparency, but is inferior in heat resistance and barrier performance and has low thermal conductivity.
In contrast, apart from the absence of optical transparency, the metal foil has excellent features such that it is superior in heat resistance, barrier performance, handling, and thermal conductivity and also has good flexibility and low material cost. In particular, a typical flexible glass or film has an extremely low thermal conductivity of 1 W/m° C. or lower, while a copper foil has an extremely high thermal conductivity of around 280 W/m° C. However, in order to use the metal foil as a supporting base material for flexible electronic devices, the surface of the metal foil needs to be covered with an insulating film.
For instance, Patent Literature 1 discloses an organic light-emitting device comprising a lower electrode layer, an organic layer, and an upper electrode layer on a flexible substrate, describing that a metal foil that may be covered with an insulating film can be used as the flexible substrate.
Patent Literature 2 discloses an insulator-coated metal foil for flexible electronic devices, which comprises a stainless steel foil having a surface roughness Ra of from 30 nm to 500 nm. The metal foil, of which the surface is coated with the insulator film, is used as a supporting base material for forming thereon an electronic device, such as thin film transistor (TFT).
Patent Literature 3 discloses a flexible metal foil laminate comprising a metal foil having a surface roughness Ra of about 0.2 μm or less and a heat-adhesive polyimide coating laminated on the surface the metal foil.
Patent Literature 4 discloses an electrolytic copper foil having a surface roughness (Rzjis) of less than 1.0 μm on the deposition side thereof so as to attain a low profile on the surface to be bonded to an insulator-layer constituent material. However, the surface roughness (Rzjis) disclosed in this literature is 0.27 μm at best.